The invention relates to a viewing screen structure formed on an interior surface of a cathode-ray tube (CRT) faceplate panel and, more particularly, to a spray shield which protects at least an interior surface portion and a seal land of the faceplate panel from a heat-absorptive overcoating sprayed on a highly reflective metal layer which overlies the viewing screen.
One type of cathode-ray tube that is used for television displays is referred to as a shadow-mask tube. This tube is comprised of an evacuated envelope having a viewing window, a viewing-screen structure comprised of a mosaic of phosphor areas (usually dots or strips) of different emission colors supported on the inner surface of the viewing window, a shadow mask having an array of apertures therein in register with the phosphor areas mounted in the tube in adjacent spaced relation with the window, and means for projecting one or more (usually three) electron beams towards the screen for selectively exciting the phosphor areas thereof.
In operating a shadow-mask tube, the electron beams are made to scan a raster in a fixed pattern. As the beams are made to scan, they are either intercepted by the mask or they pass through the mask apertures and excite the desired phosphor areas. The energy in the intercepted electron beams heats the mask and may cause the mask to become distorted, which may adversely affect the position of the beams which pass through the mask apertures. Some of the heat in the mask is removed by radiation back to a dark coating on the funnel of the tube. Normally, the viewing-screen structure includes a thin layer of a highly reflective metal, usually aluminum, which reflects heat that is radiated forward towards the screen.
U.S. Pat. No. 3,703,401 issued to S. B. Deal et al. on Nov. 21, 1972 and U.S. Pat. No. 4,025,661 issued to J. J. Moscony et al. on May 24, 1977 suggest applying to the reflective metal layer a water-based heat-absorptive overcoating of carbon particles. Then, the structure is baked to remove organic and volatile materials therefrom. The purpose of a heat-absorptive overcoating is to promote the transport of heat from the shadow mask to the atmosphere through the glass panel and thereby reduce mask warpage due to uneven heating of the mask-frame assembly of the tube. Common formulations used in applying these overcoatings include such constituents as finely-divided particles of graphite and lamp black together with dispersants and wetting agents.
The Deal et al. and Moscony et al. patents suggest air spraying the overcoating as well as an initial sealer coating or barrier layer which prevents carbon in the overcoating from penetrating through the aluminum metal layer into the phosphor mosaic. The patents also suggest the use of a suitable shield which is not described, to prevent the barrier layer and the overcoating from being sprayed on the inner sidewall of the panel and onto the seal land. It is necessary to prevent the coatings from contacting the seal land since contaminants on the seal land will adversely affect the quality of the subsequent frit seal which is required to attach the faceplate panel to the funnel portion of the tube.
In order to prevent the coatings from contaminating the seal land and from being deposited on the internal sidewall and the exterior surface of the panel it is necessary to preheat the panel in an oven to a panel temperature of about 75.degree. C. to dry the water-based coatings sufficiently rapidly to maintain high production rates. To achieve such a panel temperature, the oven temperature must be about 135.degree. C. At such a temperature the shield must be formed of a rigid plastic to maintain dimensional stability; however hard plastics provide poor sealing characteristics of the seal land and permit pull-through or leakage of the spray coating between the spray shield surface and the glass panel thereby depositing the spray coating on the interior sidewall of the panel and contaminating the seal land. Accordingly, a spray shield is required which prevents pull-through and thereby provides improved protection of the interior sidewall, the seal land and the exterior surface of the panel.